JPH04256892A - Fuel for light water reactor - Google Patents

Abstract

PURPOSE:To reduce rim effect in high burnup and attain higher burnup. CONSTITUTION:On the outer surface of pellets 1 of enriched uranium for fuel, uranium oxide 2 of natural uranium or the like with lower content of uranium 235 than the enriched uranium pellets 1 is claded and they are charged in a cladding 3. As the burning is suppressed on the surface due to the lack of fuel, temperature rise in the fuel center due to a rim effect is suppressed low and thus safety is improved and high burnup becomes possible compared with the existing fuel rods.

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to improvements in nuclear fuel. More specifically, the present invention relates to a fuel for a light water reactor type nuclear reactor.

0001

BACKGROUND OF THE INVENTION Conventionally, fuel for light water reactors is a rod-shaped product made of enriched uranium pellets covered with a cladding tube. For example, current light water reactors exclusively use fuel assemblies in which fuel rods filled with enriched uranium/UO2 pellets are bundled into Zircaloy cladding tubes. As shown in FIG. 2, the fuel rod is made by passing dried UO2 pellets 101 into a Zircaloy tube 1.
02 is filled with helium and welded closed. UO2 is made by mixing baked enriched uranium (UO2) into powder, solidifying it into pellets, and then sintering it to a high density to make highly accurate pellets.

0002

However, this conventional fuel has a problem in that as the burnup of the fuel increases, the central temperature of the fuel increases. When this fuel burns at a high rate, a phenomenon occurs in which the surface of the fuel pellet burns more than the center of the fuel pellet (rim effect). On the other hand, fuel has a property that its thermal conductivity decreases as it burns. Therefore, normally the heat generated in the fuel rods passes through the fuel pellet surface and is transferred to the cooling water around the fuel rods, but if heat conduction at the fuel pellet surface decreases due to the rim effect, heat is transferred to the cooling water. It is thought that this inhibits the conduction of fuel and increases the core temperature of the fuel. If the core temperature of the fuel becomes too high, the risk of damage to the fuel rods increases, so the increase in the core temperature of the fuel must be limited. For this reason, this is one of the factors that limits the increase in burnup of the fuel in order to limit the rise in the center temperature of the fuel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel for a light water reactor that reduces the rim effect.

0004

[Means for Solving the Problems] In order to achieve the above object, the light water reactor fuel of the present invention coats the outer surface of the fuel pellet with uranium oxide containing a small proportion of uranium-235 (235U) such as natural uranium. .

[0005]

[Operation] Therefore, since there is less fuel near the fuel surface, combustion on the fuel surface does not progress beyond the center of the fuel pellet, and the rim effect is reduced. This prevents a decrease in thermal conductivity and limits the temperature rise at the center of the fuel.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the structure of the present invention will be explained in detail based on embodiments shown in the drawings.

FIG. 1 shows an embodiment of the light water reactor fuel of the present invention. This fuel is filled into a cladding tube 3 after the surface of a fuel pellet 1 made of enriched uranium is coated with uranium oxide 2 having a low proportion of uranium 235. uranium 235
As the uranium oxide (hereinafter also referred to as coated uranium) 2 having a small proportion of uranium, it is preferable to use, for example, natural uranium.

The coated uranium 2 is coated during the process of producing enriched uranium 1 pellets. For example, after solidifying enriched uranium 1 into a pellet, its surface is coated with coated uranium 2,
Either sintered together to a high density or coated after sintering the pellets 1.

The thickness and amount of this coated uranium 2 is preferably about 200 μm to 300 μm, for example. If the amount is more than this, the heat generation rate in the fuel pellets 1 will become too high, and if it is too small, the rim effect cannot be suppressed.

[0010] In this way, when the outer surface of the fuel pellet 1 is coated with natural uranium 2 or the like, which has a lower proportion of uranium 235, which is a nuclear fuel material, than enriched uranium, there will be less fuel near the fuel surface. Therefore, combustion on the fuel surface does not progress beyond the center of the fuel, reducing the rim effect. therefore,
The fuel center temperature at high burnup is lower than that of conventional fuels,
Improves fuel rod safety. Therefore, it becomes possible to use fuel up to a higher burnup, and the reduction in power generation costs achieved by increasing the burnup is promoted, leading to improved economic efficiency.

[0011]

[Effects of the Invention] As is clear from the above explanation, in the light water reactor fuel of the present invention, coating the outer surface of the fuel pellet with uranium oxide, such as natural uranium, which has a lower proportion of uranium-235 than enriched uranium, is not a fuel-related problem. Since it does not change the diameter, it does not affect various characteristics of the reactor other than the fuel center temperature. Therefore, it can be easily used in current light water reactors without impairing the characteristics of the nuclear reactor plant, simply by improving the safety of the nuclear reactor. Moreover, in the production of fuel, it can be easily produced by simply adding a step of coating natural uranium, etc., and it is economical with little impact on production costs.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a fuel rod showing an embodiment of the fuel for a light water reactor according to the present invention.

FIG. 2 is a cross-sectional view showing an example of a conventional light water reactor fuel.

[Explanation of symbols]

1 Enriched uranium 2 Uranium oxide with a low proportion of uranium-235 3
cladding tube

Claims (1)

[Claims] 1. A fuel for a light water reactor, characterized in that the outer surface of fuel pellets is coated with uranium oxide containing a low proportion of uranium-235, such as natural uranium.